1. Field of the Invention
This invention relates to beam-addressed electrostatically-actuated light modulators and more specifically to a charge controlled mirror (CCM) that uses a thin insulating membrane to decouple the micromirror array from the beam within a vacuum.
2. Description of the Related Art
In an electrostatically-actuated light modulator, a beam of light is directed towards a light valve target that, in response to a video addressing signal, imparts a modulation onto the beam in proportion to the amplitude of the deflection of the individual reflective elements, e.g. a reflective thin-film or an array of micromirrors. The amplitude or phase modulated beam is then passed through projection optics to form the image. The target produces attractive electrostatic forces between the underlying substrate and the individual reflective elements that pull them inward toward the substrate. The amplitude of deflection corresponds to the pixel intensity in the video signal. It is well known that optical performance of the light modulator is closely tied to deflection range, electrostatic instability and resolution.
Deflection range is strictly limited by the spacing of the reflective elements above the substrate. Furthermore, it is commonly understood that only about one-third of the gap can be usefully employed due to problems of electrostatic instability. The attractive forces tend to overwhelm the restoring spring force of the reflective element, causing it to snap all the way to the base electrode. This problem is commonly referred to as pull-in or snap-over. Once the element snaps over, it remains stuck to the substrate due to the Van der Waals forces. The useful range can be extended to about four-fifths of the gap by using a control electrode underneath the element whose diagonal is about 60% of the length of the element's diagonal. However, this does increase the voltage required to achieve the same amount of deflection.
In the late 1960s, RCA developed a new Schlieren light valve that used a high energy scanning electron beam in a vacuum to address a thin metal film supported in close proximity to a glass substrate, which is described in J. A. van Raalte, "A New Schlieren Light Valve for Television Projection", Applied Optics Vol. 9, No. 10, (October 1970), p. 2225. The electron beam penetrates the metal film and deposits charge on the substrate in proportion to the intensity of the video signal. The deposited charge produces an attractive force that deforms the metal film inward towards the substrate, which causes a portion of the reflected light to miss the stop, thereby increasing screen brightness until eventually all the light reaches the screen. In actual operation, each pixel deforms parabolically. Consequently, light incident on the central portion of each pixel element is not deflected, which limits fill factor and optical efficiency. In addition, deflection range is limited to about 20% to maintain parabolic deformation.
Erasure is achieved by bleeding off the charge deposited in the dielectric layer through the faceplate of the vidicon tube. The faceplate's RC time constant can be set to insure that substantially all of the deposited charge is bled off in one frame time. However, the faceplate must be heated in order to minimize the variation in discharge times created by the traps in the dielectric material. In addition, the glass substrate will, over time, tend to discolor in response to the electron bombardment.
More recently Optron Systems, Inc., as described in Warde et al., U.S. Pat. No. 5,287,215, has developed a membrane light modulation system in which a charge transfer plate (CTP) couples charge from a scanning electron gun under vacuum through to potential wells in atmosphere. An array of insulating posts formed in or on the CTP support a deformable reflecting membrane that spans the wells. The CTP serves as a high-density multi-feedthrough vacuum-to-air interface that both decouples the electron beam interaction from the membrane and provides the structural support required to hold off atmospheric pressure. The vacuum-to-air interface allows the reflective membrane to be built and operated in air rather than a vacuum, which is simpler and cheaper.
However, because the CTP provides structural integrity sufficient to withstand atmospheric pressure, the CTP must be very thick, at least 3 mm for useful display sizes. In order to preserve the resolution of the deposited charge pattern, the rule-of-thumb is that the charge plane should be preferably within one-tenth the width of the pixel and no greater than ten times the width. At large distances, the fringing forces will washout the resolution of the attractive electrostatic forces. Even assuming a fairly large pixel size of 0.1 mm the charge plane could be no greater than 1 mm away and preferably about 10 microns. To effectively move the charge plane closer to the membrane, Warde forms conductive feedthroughs in the CTP to transfer the charge pattern from the backside of the CTP to the wells, which are nominally spaced 2-10 microns from the membrane.
Although the feedthroughs solve the proximity problem they dramatically reduce the amount of charge delivered to the wells. Since charge distributes itself uniformly around the cylindrical feedthrough and the area of one end of a feedthrough might be 1/1000 its total surface area for these dimensions, the amount of charge delivered to the well is reduced by approximately 1/1000. Thus, the scanning electron gun has to deliver approximately 1000 times the charge needed to actuate the membrane. The higher the current density the larger the beam spot size, hence the lower the resolution of the display.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays," ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,911, 3,886,310 and 3,896,338. The device is fabricated by growing a thermal silicon dioxide layer on a silicon-on-sapphire substrate. The oxide is patterned in a cloverleaf array of four centrally joined cantilever beams. The silicon is isotopically wet-etched until the oxide is undercut, leaving four oxide cantilever beams within each pixel supported by a central silicon support post. The cloverleaf array is then metallized with aluminum for reflectivity. The aluminum deposited on the sapphire substrate forms a reference grid electrode near the edges of the mirrors that is held at a d.c. bias. A field mesh is supported above the mirrors to collect any secondary electrons that are emitted from the mirrors in response to the incident primary electrons.
The device is addressed by a low energy scanning electron beam that deposits a charge pattern on the cloverleaf beams, causing the beams to be deformed toward the reference grid electrode on the substrate by electrostatic actuation. Erasure is achieved by holding the deposited charge on the mirror throughout the frame time and then raising the target voltage to equal the field mesh potential while flooding the tube with low energy electrons to simultaneously erase all of the mirrors. This approach increases the modulator's contrast ratio but produces "flicker", which is unacceptable in video applications.
To allow deformation of the cloverleaf beams with reasonable amounts of charge deposited by the scanning electron beam, Westinghouse must make the cloverleaf beams thin and pliable. Also, in order to avoid a crystal grain structure that would reduce reflectivity by 10-15%, Westinghouse must use a very thin aluminum coating on the cloverleaf beams. As a result, the electron beam energy must be relatively low so that substantially all of the electrons are stopped in the mirror and do not penetrate through to the underlying glass substrate. Unfortunately, low energy beams exhibit relatively large spot sizes, which reduces resolution.
A thicker mirror could be used in combination with a high-energy electron beam to stop the incident electrons and maintain high resolution. However, to ensure quality video performance, the fundamental mechanical resonance of the mirror must exceed the video rate by approximately a factor of one hundred to allow the mirror to be fully settled. Resonance frequencies of 5-10 kHz are suitable. As a result the hinge formed at the juncture of the cloverleaf beams and support post would have to be fairly strong, i.e. thick. Since, the spring force of the hinge, which opposes the deflection of the beam, increases as the cube of its thickness a lot more charge would have be deposited on the beam to produce adequate force. Electron guns capable of delivering sufficient current in a one-pixel dwell time with a small spot size are currently beyond the state of the art.
The persistent snap-over problems and limited contrast ratio has, to date, kept micromirror light modulators from exploiting their inherent optical power advantages over CRT, liquid crystal and reflective membrane based technologies and dominating the market. In fact, the problem has been so onerous that the light modulator industry has invested considerable time and money to develop a digital mode of operation for these attractive-mode devices and alternate actuation technologies.
Texas Instruments has pioneered the development of the digital-mode light modulator with its digital micromirror device (DMD) that uses the pull-in problem to its advantage. The DMD employs a torsional micromirror that rocks back-and-forth between binary positions with the tips of the mirror being pulled down to the base electrodes. The "sticking" effect is diminished, but not eliminated, by only touching the tip of the mirror to the base electrode and by using anti-stick coatings. Time division multiplexing (TDM), created by rapidly rocking the mirror back-and-forth between its two positions, is used to establish different gray-levels. The electronics for implementing a TDM addressing scheme arc much more complex and expensive than those required for analog modulation. Furthermore, the anti-stick coatings complicate the device and increase production costs significantly.
Aura Systems has developed a micromirror light valve target in which electrostatic actuation has been replaced with piezoelectric actuation. The micromirrors are formed on top of piezoelectric pedestals. DC voltages are applied to the pedestals, which causes them to change shape and tilt the mirrors. Although piezoelectric actuation avoids pull-in, the fabrication process is complex and expensive, the deflection angles arc small, and high voltages switched at high frequencies are required to actuate the pedestal.